Toxic responses to environmental chemicals are known to be dependent on exposure route, kinetic behavior, as well as the dosage used in the toxicology study. Therefore the knowledge of the basic kinetic parameters, bioavailability, dose proportional range, and internal dose are indispensible. Such data are vital for the interpretation of toxicology study results, for the facilitation of interspecies scaling, as well as risk assessment. Extrapolations to man can then be made so that the public can be adequately informed about potential risks from exposure to hazardous chemicals. Emodin is the active ingredient in a large number of herbal laxatives and knowledge of its basic kinetic parameters is imperative to help interpret observed toxicities. The toxicokinetics of emodin were determined in F344/N rats and B6C3F1 mice. Groups of rodents were dosed with emodin intravenously and orally with blood samples being collected at various time points after dosing. The plasma was then prepared for analysis of emodin and the glucuronide metabolite using a bioanalytical method that was developed specifically for these studies. Results of the analyses showed a relatively rapid elimination of emodin and a low bioavailability. Data was used to help select dosages for the carcinogenicity study.